Video communications over a communication network are currently being carried out using source encoders and channel encoders that were designed independently of each other. The tradition of separating the source and channel encoding tasks arises from the theoretical foundation of Shannon's separation principle of source and channel encoding that espouses the notion that this separation is optimal. According to this principle, the task of the source encoder is to do the best job it can in compressing the input signal so that the compressed bit rate will match the available bit rate of the communication channel.
Shannon's separation principal does, however, fail to address the error characteristics of the communication channel on which the video information will traverse. Importantly, developing an encoder without regard to these error characteristics has lead to the result that the image signal is rendered effectively useless if even a few bits are corrupted during transmission. This problem becomes especially pronounced in a wireless communication network where large variations and unpredictability in the error characteristics abound. Unfortunately, currently utilized video coding standards including H.261, MPEG-1, MPEG-2 and H.263, which were developed without regard to the error characteristics of the communication channel, fail to provide the error resiliency needed to minimize this problem.
Furthermore, currently utilized error protection and error recovery schemes that are employed to alleviate the noted error-induction problem have the undesirable side effect of wasting bandwidth within the communication channel. For example, a commensurate amount of bandwidth must be allocated to video transmissions at the expense of other transmissions when additional error checking bits are added to the video bit-stream. Furthermore, using an automatic repeat request (ARQ) procedure that functions to initiate the re-transmission of those transmissions deemed to be in error, while providing error recovery, adds both delay and bandwidth overhead to the transmissions. Delay for the case of interactive real-time video applications causes unacceptable application performance, while bandwidth overhead reduces the efficiency of the overall system.
From the foregoing, it is evident that a need exists for software and hardware mechanisms that ensure timely and robust delivery of video data over bandwidth-limited, error-prone communication channels.